The present invention relates to a method for producing a semiconductor device such as a semiconductor light-emitting device or a field effect transistor.
III-V nitride semiconductors typified by gallium nitride (GaN), aluminum nitride (AlN) or indium nitride (InN) (hereinafter, referred to as nitride semiconductor) are materials whose band gap is in a wide range from 1.9 eV to 6.2 eV and can cover a wavelength band from infrared light to ultraviolet light.
In general, sapphire (monocrystal Al2O3) is used as a substrate in which a nitride semiconductor is grown. Although there is large lattice mismatch between sapphire and a nitride semiconductor, high quality crystal can be obtained by providing a low temperature buffer layer between the substrate made of sapphire and the nitride semiconductor layer. As a result, a light-emitting diode element made of the nitride semiconductor formed on the sapphire substrate is commercially available at present.
Since the nitride semiconductor has a large breakdown voltage, the nitride semiconductor can be expected to be applied, not only as a semiconductor light-emitting device employing the nitride semiconductor, but also as a semiconductor device that can be operated at a large power, and can withstand high temperature operations during large power operation.
However, as described above, in the semiconductor device in which a semiconductor element is formed on a substrate made of sapphire, the characteristics of sapphire itself causes various problems.
First, a stress generated by the difference in the thermal expansion coefficient between the nitride semiconductor and sapphire adversely affects the element. This stress inevitably occurs when the nitride semiconductor layer is epitaxially grown on the substrate in a comparatively high temperature atmosphere, and then the temperature is returned to room temperature.
Secondly, sapphire has a high hardness and is chemically stable, so that processing such as etching or polishing is difficult. For example, when a wafer in which an element is formed is divided into chips by dicing, cracks are likely to occur in the divided chips, and cleavage is difficult to perform. In addition, although the substrate constitutes the major part of the volume of a chip itself, it cannot be separated nor removed, and therefore it is difficult to achieve compactness and thinness.
Thirdly, sapphire is an insulator, it is impossible to form electrodes directly on the substrate. Therefore, it is necessary to form a positive electrode and a negative electrode on an epitaxial layer, and to mount a semiconductor device by a flip-chip technique, which results in a large element area.
Fourthly, sapphire has a small thermal conductivity, so that the heat release properties from the substrate is poor and the temperature characteristics of the semiconductor device cannot be improved.
Therefore, in view of the conventional problems, it is an object of the present invention to achieve that a stress applied to a semiconductor layer from a mother substrate on which the semiconductor layer is grown can be reduced reliably, and that the mother substrate can be separated from the semiconductor layer easily.
In order to achieve the above object, the present invention provides a method for producing a semiconductor device including forming a thermally decomposed layer formed by thermally decomposing a first semiconductor layer between the first semiconductor layer grown on a mother substrate and the mother substrate before growing a second semiconductor layer including an active layer.
More specifically, a method for producing a semiconductor device of the present invention includes a first step of forming a first semiconductor layer on a mother substrate; a second step of forming a thermally decomposed layer formed by thermally decomposing the first semiconductor layer between the first semiconductor layer and the mother substrate by irradiating the mother substrate with irradiation light from the surface opposite to the first semiconductor layer; and a third step of forming a second semiconductor layer including an active layer on the first semiconductor layer in which the thermally decomposed layer is formed.
According to the method for producing a semiconductor device of the present invention, the thermally decomposed layer ensures that the stress applied from the mother substrate to the first semiconductor layer when the second semiconductor layer is formed on the first semiconductor layer and then the mother substrate provided with the second semiconductor layer is cooled back to room temperature, which is caused by the difference in the thermal expansion coefficient between the mother substrate and the first semiconductor layer, can be reduced. Consequently, defects such as cracks occurring in the second semiconductor layer including the active layer can be prevented, so that the yield can be improved.
It is preferable that the method for producing a semiconductor device of the present invention further includes a fourth step, between the first step and the second step, of forming a mask film on the first semiconductor layer, the mask layer being made of a material that substantially prevents the second semiconductor layer from being grown and having a plurality of openings.
It is preferable that the method for producing a semiconductor device of the present invention further includes a fourth step, before the first step, of forming a mask film on the mother substrate, the mask layer being made of a material that substantially prevents the first semiconductor layer from being grown and having a plurality of openings.
It is preferable that the method for producing a semiconductor device of the present invention further includes a fifth step, after the third step, of separating the mother substrate from the first semiconductor layer by removing the thermally decomposed layer.
In this case, it is preferable that the method for producing a semiconductor device of the present invention further includes a sixth step, after the fifth step, of forming an electrode on the surface of the first semiconductor layer opposite to the second semiconductor layer.
In the method for producing a semiconductor device of the present invention, it is preferable that the first semiconductor layer is made of a compound semiconductor containing a nitride.
In the method for producing a semiconductor device of the present invention, it is preferable that the second semiconductor layer is made of a compound semiconductor containing a nitride.
In the method for producing a semiconductor device of the present invention, it is preferable that the first semiconductor layer is a contact layer of the second semiconductor layer.
In the method for producing a semiconductor device of the present invention, it is preferable that the first semiconductor layer is a cladding layer of the second semiconductor layer.
In the method for producing a semiconductor device of the present invention, it is preferable that the first semiconductor layer is a compound semiconductor made of a p-type nitride.
In the method for producing a semiconductor device of the present invention, it is preferable that the irradiation energy of the irradiation light is about 0.1 J/cm2 or more and about 20 J/cm2 or less. This ensures that the mother substrate and the first semiconductor layer are attached by the thermally decomposed layer formed by thermally decomposing the first semiconductor layer.
In this case, it is preferable that the wavelength of the irradiation light is longer than the absorption edge of the forbidden band of a material constituting the mother substrate and is shorter than the absorption edge of the forbidden band of a material constituting the first semiconductor layer.